mediumMultiple Choice
350-401 Practice Question: Consider the following configuration snippet:…
Consider the following configuration snippet:
policy-map QOS_POLICY
class VOICE
priority percent 30
class VIDEO
bandwidth percent 20 queue-limit 50 packets
class class-default
fair-queue queue-limit 100 packets
What is the effect of this configuration?
⚠ Common exam trap
Cisco often tests the misconception that excess priority traffic is re-queued into the default class or another queue, when in fact it is always dropped to enforce the bandwidth limit and protect other traffic classes.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The VOICE class traffic is always sent before other classes, but if it exceeds 30% of the interface bandwidth, excess traffic is dropped.
The 'priority percent 30' command under the VOICE class enables strict priority queuing, meaning VOICE traffic is always transmitted before any other class. However, the priority queue is policed at 30% of the interface bandwidth; any traffic exceeding this rate is dropped, not queued. This is a fundamental behavior of the priority command in Cisco IOS — excess priority traffic is dropped to prevent starvation of other queues.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
The VOICE class traffic is always sent before other classes, but if it exceeds 30% of the interface bandwidth, excess traffic is dropped.
Why this is correct
The VOICE class is assigned strict priority through the `priority` command, so its packets are dequeued before any other class whenever the priority queue is non-empty. The `percent 30` keyword configures an aggregate policer that allows traffic up to 30% of the interface bandwidth and drops any excess immediately (conform-action transmit, exceed-action drop). This is a hard ceiling—excess voice traffic is not re-queued or forwarded, it is discarded on the spot.
- ✗
The VOICE class traffic is always sent before other classes, and excess traffic beyond 30% is queued in the default class.
Why it's wrong here
This option incorrectly assumes that excess priority traffic is reclassified and moved into the default class. In Cisco MQC, the priority command with a percent policer uses a token-bucket policer: conforming traffic is transmitted, and exceeding traffic is dropped immediately—there is no mechanism to transfer excess frames to class-default. The default class only receives traffic that was never matched by a user-defined class, not overflow from a priority class.
- ✗
The VIDEO class traffic is treated with strict priority after the VOICE class.
Why it's wrong here
Only the VOICE class has the `priority` command in this policy, so it is the sole class that receives strict-priority treatment. VIDEO is configured with a bandwidth guarantee (e.g., `bandwidth percent 20`), which provides a minimum bandwidth allocation under congestion, not priority forwarding. Even if VIDEO had a priority command, it would be serviced after VOICE because VOICE is also a priority class; but here VIDEO lacks the priority keyword entirely.
- ✗
The class-default uses Weighted Fair Queuing with a maximum queue size of 100 packets, and all classes share the remaining bandwidth equally.
Why it's wrong here
The class-default does run fair-queue by default, but the statement that it has a maximum queue size of 100 packets is not part of the MQC policy shown; queue depth is governed by platform buffer settings, not a fixed 100-packet limit. More importantly, bandwidth is not shared equally among all classes: VOICE and VIDEO each have their own allocated bandwidth percentages, and class-default only receives the remaining bandwidth, which is then further divided by fair-queue among flows inside that class. VIDEO is guaranteed a fixed 20% of the interface bandwidth, not an equal share of the remaining bandwidth.
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